(1) Field of the Invention
The present invention relates to an ink composition for an aqueous ink ball point pen which does not cause feathering on written lines and does not allow an ink to strike through into a back side of paper and which does not bring about starving, blobbing and splitting and makes it possible to write fine letters.
(2) Related Art
Low viscosity aqueous ink ball point pens have so far made a smooth feeling in writing and enabled to write with a low pen pressure but caused feathering on written lines.
Then, for the purpose of solving the feathering problem, a viscosity controlling agent (gelling agent) for providing an ink with a shear thinning property (non-Newtonian property) is added to an ink composition for an aqueous ink ball point pen.
Known as the above ink composition for an aqueous ink ball point pen are, for example, 1) an ink composition for an aqueous ink ball point pen characterized by containing 0.20 to 0.45% by weight of xanthan gum (Japanese Patent Application Laid-open Sho 59 No. 74175) and 2) an aqueous ink composition for a writing instrument suitable for a ball point pen, comprising as essential components, a) a colorant, b) an organic acid-modified heteropolysaccharide which comprises a fundamental unit constituted by glucose/galactose/pyruvic acid or a salt thereof/succinic acid or a salt thereof/acetic acid of 5 to 8/1 to 2/0.5 to 2/0.5 to 2/0.5 to 1 in terms of a molar ratio and which has an average molecular weight of about 1,000,000 to about 8,000,000 and c) an aqueous medium comprising water and a water-soluble organic medium, wherein water accounts for 50 or more % by weight (Japanese Patent Application Laid-open Hei 6 No. 88050).
The ink compositions for an aqueous ink ball point pen described in Japanese Patent Application Laid-open Sho 59 No. 74175 and Japanese Patent Application Laid-open Hei 6 No. 88050 described above can be improved in feathering on written lines, but involved therein are problems that they bring about splitting and blobbing and that they are short of an ink followability and do not make a smooth feeling in writing.
On the one hand, for example, an aqueous pigment ink comprising at least titanium oxide, an extender pigment, a dispersant, an emulsion of polyvinyl alcohol having a polymerization degree of 200 to 1000 and/or a styrene•acryl base copolymer and water (Japanese Patent Application Laid-open No. 2000-345090) is known as an ink for a writing instrument such as a marking pen and a ball point pen, in which polyvinyl alcohol is used. Also, for example, an aqueous pigment ink comprising at least a pigment, anionic group-modified polyvinyl alcohol and water (Japanese Patent Application Laid-open Hei 5 No. 271597) is known as an aqueous pigment ink, in which modified polyvinyl alcohol is used.
On the other hand, as an aqueous ink composition in which an alkali metal borate and the like are used, known are an aqueous ink composition comprising an alkali metal silicate and/or an alkali metal borate for improving corrosion resistance of a cemented carbide-made ball at a tip part of the pen, and an aqueous ink ball point pen charged with the same (Japanese Patent Application Laid-open No. 2002-338869).
However, the polyvinyl alcohol in the ink for a writing instrument described in foregoing Japanese Patent Application Laid-open No. 2000-345090 is used in order to improve dispersion stability of the pigment in freezing and thawing, and the objects and the problems are different from those in the present invention. In addition thereto, it is neither described nor perceived at all to use it in combination with borate.
Also, the anionic group-modified polyvinyl alcohol in the aqueous pigment ink described in foregoing Japanese Patent Application Laid-open Hei 5 No. 271597 is used for an aqueous pigment ink which is less liable to bring about the problem of wrinkles and warps on paper (Japanese writing paper) after drying handwritings and which is suitable particularly for Indian ink, and the objects and the problems are different from those in the present invention. In addition thereto, it is neither described nor perceived at all to use it in combination with borate.
Further, the alkali metal borate in the aqueous ink composition described in foregoing Japanese Patent Application Laid-open No. 2002-338869 is used in order to form a thin film of the alkali metal borate on the ball to prevent the ball and the tip holder from being corroded, and the objects and the problems are different from those in the present invention. In addition thereto, it is neither described nor perceived at all to use the alkali metal borate in combination with polyvinyl alcohol.